Haemoglobin is a protein that transports oxygen from the lungs of vertebrates to the rest of their body.

Garvey says oxygen molecules are taken up by, and released from, iron atoms in the haemoglobin and this is understood to happen most efficiently when the haemoglobin molecule partly unfolds and softens.

He and colleagues studied the way haemoglobin makes this transformation in different species with different body temperatures.

"We've been looking at how its softness changes as a function of temperature," says Garvey.

The researchers studied haemoglobin from three species that regulate their own body temperatures, known as endotherms. These were humans, the platypus, which has the lowest body temperature of vertebrates at 34°C, and the domestic chicken, which has a high body temperature of 42.8°C.

They used various techniques, including neutron scattering to see how soft the molecules were, and computer simulations to investigate the amino acid sequences determining the softness of the molecule at different temperatures.

Garvey and colleagues found that at the body temperature of each organism, haemoglobin's softness changed abruptly.

"At the body temperature it goes from being quite rigid to being rubbery," says Garvey.

Amino acid sequences

While haemoglobin from each animal had a similar molecular structure, the researchers found the temperature at which it became softest was influenced by amino acids on either side of small holes in the haemoglobin, which are known to play a role in regulating the absorption of oxygen.

The researchers also looked at haemoglobin from the salt water crocodile, which is an ecotherm and does not regulate its temperature, and found it did not undergo the same transition in softness.

Garvey says the findings suggest that the transition in how soft haemoglobin is at different temperatures, is an evolutionary adaptation in endotherms, although he is not sure what evolutionary pressure would have led to this.

Emeritus Professor Gordon Grigg of the University of Queensland who has studied oxygen transport in crocodiles welcomes the research.

"It's exciting work and offers all sorts of possibilities," he says. "Perhaps it's opening up a new way to look at the evolution of haemoglobin."

He says the data at present is insufficient to draw any strong conclusions from, and suggests a useful next step would be for the researchers to work with comparative physiologists to explore this question.